1. Field of the Invention
The technology disclosed relates to memory devices based on phase change memory materials, like chalcogenides, and on other programmable resistance materials, and methods for manufacturing such devices.
2. Description of Related Art
In some programmable resistance memory array configurations, each memory cell includes a diode in series with a programmable resistance memory element. The diode acts as an access device, so that memory cells can be selected for operation by forward bias on the diode, while current flow in unselected memory cells is blocked by reverse bias on the diode. Thus, the biasing arrangements used in these configurations to access selected cells are set up to reverse bias the diodes in unselected cells. Some configurations utilize transistors for access devices, which are also subject to biasing arrangements for selected and unselected cells.
One type of programmable resistance memory element comprises phase change material that exhibit a large resistivity contrast between crystalline (low resistivity) and amorphous (high resistivity) phases. Phase change materials may include chalcogenides, and other alloys of materials such as germanium (Ge), antimony (Sb), tellurium (Te), gallium (Ga), indium (In), silver (Ag), selenium (Se), thallium (Ti), bismuth (Bi), tin (Sn), copper (Cu), palladium (Pd), lead (Pb), sulfur (S), and gold (Au). In normal operation of a phase change memory element, an electrical current pulse passed through the phase change memory cell can set or reset the resistivity phase of the phase change memory element. To reset the memory element into the amorphous phase, an electrical current pulse with a large magnitude for a short time period can be used. To set the memory element into the crystalline phase, an electrical current pulse with a medium magnitude and a longer time period can be used. To read the state of the memory element, a small voltage is applied to the selected cell and the resulting electrical current is sensed. The sensed current may have at least two current levels, one very low for a high resistance state, and another higher for a low resistance state. Accordingly, an electrical current is used when an operation is needed to set, reset or read the resistivity state of a selected memory cell. During such operations, it is desirable that no current passes through the unselected memory cells, because undesired current flow might cause disturbance of the data values stored in the unselected cells, or contribute leakage current that can interfere with a successful read of the selected cell or cells. However, a defective access device can have a permanent ON state, or leaky state, and conduct an electrical current even under biasing applied to unselected cells. The resulting current leakage increases the power consumption of the memory array in a memory chip, and can reduce the electrical current that reaches the target cells for reset, set or read operations in the memory array.
Programmable resistance elements other than phase change memory elements, such as metal oxide memory elements, solid state electrolyte (conductive bridge) memory elements, magnetoresistive memory elements, and so on, involve biasing adapted for their respective characteristics, and are susceptible to current leakage as well, when access devices are defective.
It is desirable to provide technology which can block current leakage and reduce power consumption in a memory array caused by defective selecting devices such as shorted diodes.